The present invention is related to an improved process for the manufacture of halogen-substituted N-allylamides from halogen-substituted O-allylimidates using palladium transition metal catalysts.
N-allylamides are useful as pesticides and as intermediates in the preparation of pesticides. A method for synthesizing these amides is by a Claisen rearrangement of O-allylimidates. Unfortunately, the Claisen method requires very high temperatures, typically of 200.degree. C. and above. Also, when a halogen-substituted methyl group is present on the 2-carbon of the imidate, use of the method results in a mixture of compounds but no desired product. The high temperature required for the rearrangement appears to destroy the starting material.
A procedure has been reported which uses transition metals, including zero-valent palladium (Pd(O)) or two-valent palladium (Pd(II)), as catalysts in Claisen rearrangements.
Schenck et al. (JACS, 1985, 107: 2058-2066) discuss the use of Pd(O) and Pd(II) catalysts, in addition to other transition metals, on a variety of allylimidate compounds, none of which imidates include a halogen-substituted methyl at the 2-position. The data in the article indicate that there are major differences between the Pd(O) and Pd(II). The reaction runs much more slowly with the Pd(II) than with the Pd(O). Also, reaction with Pd(II) always gives products with the desired regioselectivity (i.e., a [3.3]-sigmatropic shift) whereas reaction with Pd(O) produces either a mixture of Claisen ([3.3]-sigmatropic) and anti-Claisen ([1.3]sigmatropic) products or all anti-Claisen product, apparently due to the presence of a .pi.-allyl intermediate. The data also indicate that the presence of a substituent on the 2-carbon has a marked effect on the velocity of the reaction; it slows the reaction down considerably.
Ikariya et al. (Chemistry Letters, 1982, pp. 1815-1818 ) discuss the use of Pd(O) and Pd(II) as catalysts in the rearrangement of O-allyl-N-phenylformimidates to give N-allyl-N-phenylformamides. The only substituents shown on the 2-carbon are hydrogen and phenyl. Halogen-substituted methyl groups at that position are not disclosed. Ikariya et al. agree with Schenck et al. that the catalytic activity of Pd(O) is much higher than that of Pd(II) and that Pd(II) is regioselective while Pd(O) is not. In contrast to Schenck et al., Ikariya et al. note an apparent slight acceleration of the rearrangement when a substituent, here phenyl, is present on the 2-carbon of the imidate.
Overman (Angew. Chem. Int. Ed. Engl., 1984, 23: 579-586) discusses generally allylic ester rearrangements using Hg(II) and Pd(II) salts as catalysts. Rearrangement of imidates is not discussed in the article. Tamaru et al. (J. Org. Chem., 1980, 45: 5221-5223) address only palladium-catalyzed rearrangement of S-allylthioimidates to give N-allylthioamides and observe that while Pd(II) gave the desired product. Pd(O) did not. The thiono-thiolo allylic rearrangement of O-allyl phosphoro- and phosphonothionates by catalysis with Pd(O) is described by Tamaru et al. in J. Org. Chem., 1983, 48: 1293-1297. This reaction gave mainly products in which the sulfur atom was bonded to the least substituted carbon atom, regardless of the substitution pattern of the allylic groups. Thus, for example, O-crotyl was converted to S-crotyl rather than to S-(1-methyl)allyl. Contrary to other reports, in this case Pd(II) was generally not effective.